Airbags consist of frames in which there is fixed a folded bag which inflates rapidly by means of a gas produced by a generator when specific sensing devices detect that the vehicle has been involved in a collision. The bag is thus deployed, protecting the driver or a passenger, depending on his or her location, and prevents their bodies from impacting against any part of the vehicle.
When an airbag is activated, the main function is to restrain the occupant by exerting force against him or her which produces a deceleration. The airbag must maintain a certain degree of pressure during the collision of the occupant, and this pressure must be comprised between certain limits which allow complying with biomechanical parameters assuring the safety of the occupants, because otherwise the surface may be too hard and the occupants will bounce off, or it will be too soft and the occupants will impact against the vehicle.
The pressure of the bag depends on the amount of gas it houses inside. Once the airbag is activated, the generator expels all the gas, the only manner of controlling the amount of gas in the bag is by removing it.
Different proposals in this sense are known such as:                The use of more or less permeable fabrics for the bag of the airbag.        The demarcation of outlet areas by means of seams which break when a specific pressure or temperature is reached, leaving an area free for discharging the gases.        The incorporation of venting openings for discharging the gas, in some cases provided with different means of sealing the opening to achieve a better control of the internal pressure in the bag than that provided by the variation of the size of the opening, for example. Fabrics burning at a specific temperature or resilient membranes breaking at a specific pressure can be mentioned among these means. Patent applications WO02/096721, WO02/14115, and WO2004/056618 of the same applicant of the present invention belong to this category.        
However, for the case of airbags in which small venting areas are required or for the case in which it is desirable to vary the venting area using the same sealing means, the mentioned proposals do not solve the problem of suitably controlling the pressure of the gas inside the bag.
The present invention is aimed at solving this problem.